A compound will have the same composition by mass.
Calculate the following percentage by mass:
percent = portion/total x 100
If two samples are the same substance,
they should have the
same percent composition by mass.
A compound will have the same physical and chemical characteristics.
Do the have the same percent composition? Are they the same compound?
Mass Ratio
Percent Composition of compounds
was used to determine
the MASS RATIO between elements.
Regardless of number of atoms,
the
MASS RATIO
is the same.
carbon-12
The standard for mass was arbitrarily
chosen to be one atom of carbon-12
the MASS RATIO between elements
was then used to calculate others.
The mass of one carbon-12 atom was
set at 12 (exactly) “amu”.
“amu” is an arbitrarily chosen amount.
ATOMIC MASS UNIT
“amu” is an arbitrarily chosen amount.
It is so small, it is not practical for
actual use…..
A person named Avogardo,
invented a multiple that would allow
the MASS RATIO to be preserved but
to be measured in grams.
He called it : a mole
P310-317
The Mole
A mole of any element is defined to be
the same number of atoms of that
element equal to the number of atoms
in 12.0 g of carbon-12
P310-317
Molar Mass
The mass of 1 mole (in grams)
• Equal to the numerical value of the
average atomic mass (get from periodic
table)
1 mole of C atoms
=
12.01 g
1 mole of Mg atoms
=
24.31 g
1 mole of Cu atoms
=
63.55 g
Where are these numbers from?
PERIODIC TABLE
Other Names Related to Molar Mass
• Atomic Mass
Single atom, mass is measured in amu’s
instead of grams.
Only the units are different:
Atomic/Individual/MICROSCOPIC = amu
Molar/A lot of/VISIBLE/MACROSCOPIC = g
Learning Check!
Find the molar mass
(usually we round to the hundreds place)
a. 1 mole of Br atoms =
79.90 g/mole
b. 1 mole of Sn atoms =
118.71 g/mole
Molar Mass of Molecules and
Compounds
Mass in grams of 1 mole equal numerically to
the sum of the atomic masses
1 mole of CaCl2
= 110.98 g/mol
(1 mole Ca x 40.08 g/mol )
+ (2 moles Cl x 35.45 g/mol) =
110.98 g/mol CaCl2
Molar Mass of Molecules and
Compounds
Mass in grams of 1 mole equal numerically
to the sum of the atomic masses
1 mole of N2O4
= 92.02 g/mol
Learning Check!
A. Molar Mass of K2O =
B. Molar Mass of antacid Al(OH)3 =
K2O = 94.20 g/mol
Al(OH)3 = 78.03 g/mol
Other Names Related to Molar Mass
• Molecular Mass/Molecular Weight:
Single molecule, mass is measured in
amu’s instead of grams.
same numerical value as 1 mole of molecules.
Only the units are different:
Individual/MICROSCOPIC= amu
A lot of/VISIBLE/MACROSCOPIC= g
Other Names Related to Molar Mass
• Molecular Mass/Molecular Weight:
Single molecule, mass is measured in amu’s
instead of grams.
But, the molecular mass/weight is the
same numerical value as 1 mole of molecules.
Only the units are different
• Formula Mass/Formula Weight:
Same goes for compounds. But again, the numerical
value is the same. Only the units are different.
Other Names Related to Molar Mass
• THE POINT: You may hear all of these terms
which mean the SAME NUMBER… just different units
• Molar Mass = Gram Formula Weight in our textbook
• With Avogadro’s #, you may also hear the terms:
molecules, atoms, formula units, ions, particles,
“things”
IT DEPENDS ON WHAT YOU’RE TALKING ABOUT:
example: 1 dozen eggs versus 1 dozen roses
1 mole of H2O molecules vs. 1 mole of NaCl formula
units
Learning Check
Prozac, C17H18F3NO, is a widely used
antidepressant that inhibits the uptake of
serotonin by the brain. Find its molar mass.
309.38 grams
Calculations with Molar Mass
molar mass
Grams
Moles
Do you multiply or divide by molar
mass?
It depends on which way you’re going.
Use conversion factors.
Converting Moles and Grams
Aluminum is often used for the
structure of light-weight bicycle
frames. How many grams of Al are in
3.00 moles of Al?
3.00 moles Al
? g Al
1. Molar mass of
1 mole Al = 26.98 g Al
2. Conversion factors for Al
26.98 g Al
or
1 mol Al
1 mol Al
26.98g Al
3. Setup 3.00 moles Al
Answer
x
= 81.0 g Al
26.98 g Al
1 mole Al
=
Converting Moles and Grams
Iron (III) oxide is often called rust, it
forms when iron metal reacts with
moisture. How many moles of Fe2O3
are in 3.00 grams of Fe2O3?
3.00 grams of Fe2O3?
moles Fe2O3
1. Molar mass of Fe2O3 YOU MUST CALCULATE
IT!
2. Conversion factors for Fe2O3
? g Fe2O3
1 mol Fe2O3
or
1 mol Fe2O3
? g Fe2O3
3. Setup 3.00 grams Fe2O3 x
Answer
1 moleFe2O3
? grams Fe2O3
= 0.0188 mol Fe2O3
1. Molar mass of Fe2O3 159.70 grams
2. Conversion factors for Fe2O3
159.70 g Fe2O3
1 mol Fe2O3
or
3. Setup 3.00 grams Fe2O3 x
Answer
1 mol Fe2O3
159.70 g Fe2O3
1 moleFe2O3
159.70 g Fe2O3
= 0.0 188 mole Fe2O3
Learning Check!
The artificial sweetener aspartame
(Nutra-Sweet) formula C14H18N2O5 is
used to sweeten diet foods, coffee
and soft drinks. How many moles of
aspartame are present in 225 g of
aspartame?
Setting it up:
225 g C14H18N2O5 = ? mol C14H18N2O5
225g x _?__mol__ = mol
?
g
Do you know a relationship
between moles and grams
for C14H18N2O5 ?
Setting it up:
225 g C14H18N2O5 = ? mol C14H18N2O5
The relationship is: MOLAR MASS!
1 mol = the sum of the atomic masses from the periodic table in grams!
225g x _1__mol__ =
?
g
? mol
Setting it up:
225 g C14H18N2O5 = ? mol C14H18N2O5
225g x _1__mol__ =
? mol
294.34 g
How do you enter this in your calculator?
Enter first number first,
multiply numerators,
divide by denominators.
225 x 1 / 294.35 =